The invention relates generally to the field of electrical power converters and inverters. More particularly, the invention relates to techniques for predicting the life cycle of power modules, such as those used in motor drives.
Power inverters and converters employ power modules to produce a desired power waveform to power various devices, such as motors and other equipment. Power modules generally include power semiconductor switches such as insulated gate bipolar transistors (IGBTs) that are caused to cycle rapidly on and off to produce a desired power waveform. After a period of use, however, power modules tend to fail.
A predominant cause of power module failure arises from power cycling, which may cause connections to fatigue and fail. Moreover, thermal cycling of the type employed by inverters may initiate wire crack growth at aluminum wire wedge bonds and similar contact points, generally occurring at connections to aluminized silicon on the IGBTs. Strain and fatigue introduced by a mismatch of the coefficient of thermal expansion (CTE) of module materials tends to cause wire crack growth. New cracks may generally be caused by thermal cycling, leading to fatigue of the bonds of materials of the assembly.
Power modules tend to fail in a predictable manner under constant operating conditions, such as constant maximum junction temperature and median junction temperatures. As a result, manufacturers of power modules may provide cycle life rating data at particular operating conditions. For example, a manufacturer may provide various cycle life rating data at certain maximum junction temperatures (TjMax), junction temperature change (ΔTj), and mean junction temperatures (Tm). The mean junction temperature may be defined as the junction temperature averaged over time or averaged based upon the maximum and minimum junction temperature.
While power module life cycle data may offer predictability under constant operating conditions, many power module applications, such as inverter drives, tend to operate under variable conditions. Algorithms have been developed to estimate cycle life, but the algorithms generally assume constant junction temperature change (ΔTj) and constant mean junction temperatures (Tm). Moreover, though strain energy has been applied in models estimating solder joint reliability, such models do not apply directly to bond wire joints and further fail to account for variable operating conditions such as variable junction temperature (Tj) and variable mean junction temperature (Tm).